1. Field of the invention
The present invention relates to a liquid crystal display device having wide viewing angle characteristics and showing excellent color reproducibility over a wide range of viewing angle.
2. Related Art
Displays employing liquid crystal display device (also referred to as liquid crystal display panel), electroluminescence device (classified into organic types and inorganic types depending on fluorescent materials to be adopted, referred to as EL device, hereinafter), field emission device (referred to as FED device, hereinafter) and electrophoretic device, can display images, without a space (vacuum tube) such as a cathode ray tube (CRT), which is necessary for two-dimensional scanning of electron beam, on the back side of the display screen. These display devices are, therefore, characterized by their thinness, light weight, and low power consumption as compared with a display device employing a CRT. These display devices are occasionally called flat panel displays, based on the feature of their appearances.
By virtue of the above-described advantages over cathode ray tubes, the display devices employing liquid crystal display device, EL device or field emission display device are becoming widely disseminated in various applications including office automation equipment such as monitors of notebook-type personal computers and other personal computers, mobile terminals and television sets, in replacing a display device employing a CRT. Accelerated displacement of CRT-displays with flat panel displays is supported by technical innovation which relates to improvement in image quality, such as viewing angle characteristics and expansion of display range ensuring color reproducibility. In recent years, improvement has also been made on display performance of motion picture, driven by dissemination of multi-media and the Internet. The flat panel display are also launched in fields of electronic paper and large-sized information display for public use or advertisement, which could not have been realized by CRT-displays.
A liquid crystal display device generally comprises a liquid crystal cell, a drive circuit applying display signal voltage to the liquid crystal cell, a back light, and a signal control system sending input image signal to the drive circuit, wherein these are collectively referred to as a liquid crystal module.
A liquid crystal cell generally comprises liquid crystal molecules, two substrates encapsulating and holding them in between, and an electrode layer applying voltage to the liquid crystal molecules, and polarizing plate(s) disposed on the outer side thereof. A polarizing plate generally comprises a protective film and a polarizing film, and is obtained by dying the polarizing film made of a polyvinyl alcohol film with iodine, stretching the film, and then stacked with protective films on both surfaces thereof. In a transmission-type liquid crystal display device, the polarizing plates are attached on both sides of the liquid crystal cell, wherein one or more optical compensation sheets are occasionally disposed. In a reflection-type liquid crystal display device, it is general to dispose a reflection plate, the liquid crystal cell, one or more optical compensation sheets and a polarizing plate in this order. The liquid crystal cell takes part in ON-OFF display based on difference in the state of alignment of the liquid crystal molecules, and is applicable to any of transmission-type, reflection-type, and semi-transmission-type displays.
Use of the optical compensation sheet, designed to have optical characteristics optimized at each wavelength of light, can provide a liquid crystal display device less causative of color changes depending on the viewing angle. In these conventional liquid crystal display devices, trials have been made on improvement in the color changes, by controlling wavelength dispersion of retardation of the optical compensation film (Japanese Laid-Open Patent Publication No. 2002-221622). Another trial is made on balancing displayed color, by adjusting in-plane retardation values (Re) of the color filters (Japanese Laid-Open Patent Publication No. H5-196931).
The above-described liquid crystal display devices, however, have failed in improving the color changes over the entire wavelength range, and in improving coloration in oblique views.